1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT), and more particularly to a so called helical scan imaging in which a body to be examined is moved along its body axis during the scanning operation in such an X-ray CT.
2. Description of the Background Art
Recently, there has been a proposition of an X-ray CT apparatus capable of carrying out a so called helical scan imaging. As shown in FIG. 1, in the helical scan imaging, a body to be examined P located on a bed plate 100 is moved along a direction of its body axis while an X-ray tube 101 and a detector 102 is rotated around the body to be examined P, such that the X-ray tube 101 moves along a helical trajectory 103 shown in FIG. 2 relative to the body to be examined P. In reconstructing the image from the data collected by such a helical scan, tomographic image data are obtained from data collected during one rotation around the body P to be examined, such as those collected between points a and b shown in FIG. 2. Such a helical scan imaging has an advantage that the three-dimensional information on the body P to be examined can be obtained in a relatively short period of time.
Now, in such a helical scan imaging, the slice plane obtained from data collected between the points a and b does not appear like a normal slice plane shown in FIG. 3A which can be obtained by ordinary scans, but appears as shown in FIG. 3B in which 0.degree. plane and 360.degree. plane do not coincide with each other. Hence, when these data are directly used in reconstructing the image, the strong artefacts appear on the reconstructed image. For this reason, the reduction of the artefacts is achieved by deriving the data of the same single slice plane from the collected data by using the interpolation as follows.
For example, as shown in FIG. 4, the data at a point C of a desired rotational phase on a desired slice plane can be obtained by using the interpolation of the data d.sub.A of the point A in the same rotational phase as that of the point C and on a part of the trajectory 103 neighboring the point C, and the data d.sub.B of the point B in the same rotational phase as that of the point C and on another part of the trajectory 103 neighboring the point C. Therefore, in a case of using a linear interpolation, the data d.sub.C at the point C can be obtained by the following expression: ##EQU1## where l is a distance between the points A and C, and m is a distance between the points B and C, as shown in FIG. 4.
Now, as shown in FIG. 5, in reconstructing the image from the data collected by the helical scan, in order to obtain the necessary data for reconstructing the image at a slice center position E, the data must be collected at least at a main data region D which covers a half rotation (180.degree.) ahead and a half rotation (180.degree.) behind the slice center position E in a case of a full scan.
In addition, in the case whose data for reconstruction are to be derived from the collected data by using the interpolation, the data must also be collected at supplementary data regions F and G which cover a half rotation (180.degree.) ahead and a half rotation (180.degree.) behind the main data region D. Namely, in order to obtain the data at a point C' on the slice center position E by the interpolation, the data at a point A' in the main data region D as well as the data at a point B' in the supplementary data region F become necessary. Also, in order to obtain the data at a point C" on the slice center position E by the interpolation, the data at a point A" in the main data region D as well as the data at a point B" in the supplementary data region G become necessary.
Therefore, the operator must position the body to be examined P and the bed plate 100 and set up the scanning region such that the scan and the data collection can be carried out for the main data region D and possibly also for the supplementary data regions F and G if necessary, according to the desired imaging regions on the body to be examined P.
Moreover, it is further preferable for the operator to position the body to be examined P and the bed plate 100 and set up the scanning region such that the scan also covers the regions for the initial acceleration and the final deceleration of the motion of the bed plate 100 along the body axis of the body to be examined P at which the data collection is unnecessary, so as to obtain the accurate data collected only while the bed plate 100 is moving at a constant speed.
However, in a conventional X-ray CT apparatus capable of carrying out the helical scan imaging, the operator must carry out the initial set up operation including the positioning of the body to be examined P and the bed plate 100 and setting up of the scanning region described above, on his own discretion. Hence, these positioning and setting up operations are cumbersome and not very accurate.